Multi-layer composite ultrasonic transducer arrays

ABSTRACT

A piezoelectric transducer chip comprising a plurality of transducer elements. At least one of the transducer elements is a multi-layer element which comprises a plurality of piezoelectric layers, each of which is separated from the adjacent piezoelectric layers by an electrode layer so that a plurality of capacitive elements is electrically connected in parallel. The plurality of piezoelectric layers are comprised of a plurality of piezoelectric members separated from adjacent members by a gap to form a composite piezoelectric layer. Also disclosed is a method of making a multilayer transducer of a composite piezoelectric material. Alternate embodiments include two-dimensional ultrasonic transducer arrays having multilayer elements formed of a composite piezoelectric material.

This application is a continuation-in-part of U.S. application Ser. No.07/962,455 filed Oct. 16, 1992 now U.S. Pat. No. 5,329,496.

FIELD OF THE INVENTION

This application relates generally to the fields of medical diagnosticultrasound, underwater acoustic imaging, and the associatedpiezoelectric transducers.

BACKGROUND OF THE INVENTION

Diagnostic ultrasound is an essential modality in virtually everymedical specialty and particularly in obstetrics, cardiology andradiology. The ultrasound transducer is the critical component and thelimiting factor affecting the quality of diagnostic ultrasound imagingand Doppler measurements. In a conventional circular pistonpiezoelectric transducer used in mechanical scanning for medicalapplications (e.g., 19 mm diameter, 3.5 MHz resonant frequency) theelectrical impedance of the transducer is approximately 50Ω. Such atransducer is well matched to the conventional electrical transmitcircuit for delivering large amounts of acoustic power to the tissueload during the transmit mode. In a like manner, in receive mode, such atransducer is well suited for driving the typical 50Ω or 75Ω coaxialcable connected to the amplifier circuits of the scanning system.

The most sophisticated medical ultrasound scanners now typically use(N×1) linear arrays containing over a hundred transducer elements whichmay be multiplexed and/or electronically steered and focused via phasedarray techniques. A linear array transducer for phased array scanning,typically employs much smaller array elements than the conventionaltransducer described above. For example, a typical linear array includes128 elements, each of which is 0.2 mm wide by 10 mm long with a resonantfrequency of 3.5 MHz. Each piezoelectric ceramic transducer element in alinear array acts as a capacitor of approximately 150 picofarads (pf),which produces an electrical impedance Z=R_(a) +jX where X≈300Ω at 3.5MHz. At resonance the reactive component is in series with the radiationresistance R_(a), which is roughly the same magnitude as X. These higherimpedance elements reduce the sensitivity of the transducer for medicalultrasound scanning. The higher impedance element creates an impedancemismatch with conventional transmit circuitry in transmit mode, thusreducing acoustic power transmitted into the patient's body. In receivemode, the high impedance array element suffers significant losses whentrying to drive conventional coaxial cable characterized by itscapacitance/length.

It has been a significant challenge for the ultrasound community todesign and fabricate linear phased arrays for medical ultrasound overthe past two decades. Three performance characteristics have establishedconventional size and geometry of the transducer array elements: (1) theelements have sufficient angular sensitivity to steer the phased arrayover a ±45° sector angle; (2) The arrays suppress grating lobe artifactsby fine inter-element spacing; and (3) the width of each rectangularelement is small compared to the transducer thickness to removeparasitic lateral mode vibrations from the desired transducer pass band.Adherence to these performance characteristics have produced lineararrays having long narrow elements which are sized to be less than onewavelength wide for the ultrasonic frequencies used in tissue imaging,(e.g., <0.3 mm wide ×10 mm long at 3.5 MHz).

Two dimensional (N×M) transducer arrays are believed to hold promise inimproving clinical image quality in future diagnostic ultrasoundequipment. An immediate clinical application of 2-D phased arrays is thereduction of image slice thickness by focusing in the elevation planeperpendicular to the scanning dimension. An additional application of2-D transducer arrays is the correction of phase aberrations introducedacross the transducer aperture by tissue inhomogeneities. Theseaberrations occur in two dimensions, so 2-D arrays combined with theproper phase correction signal processing can restore diagnostic imagequality. In addition to improving conventional ultrasound B-scan imagequality, two-dimensional transducer arrays should assist in thedevelopment of new modes of ultrasound imaging. Projected new techniquesinclude: (1) presentation of simultaneous orthogonal B-mode scans; (2)acquisition of several B-scans electronically steered in the elevationdirection; (3) development of high-speed C-scans; and (4) high-speedvolumetric ultrasound scanning to enable real time three-dimensionalimaging and volumetric, angle-independent flow imaging. With knowntechnology, these techniques can only be implemented with 2-D arraytransducers.

Unfortunately, the design and fabrication problems of one-dimensionaltransducer arrays become almost overwhelming when extended to a twodimensional array, in which case the element size may be less than 0.2mm ×0.2 mm for more than 1000 elements in the array. There are twosignificant obstacles which limit the use of 2-D transducer arrays.First, a simple fabrication method for the electrical connections tosuch array elements, which can be less than one ultrasound wavelength ona side, is not known. Second, it is very difficult to achieve adequatesensitivity and bandwidth from such small elements.

In the last 15 years there have been several descriptions of prototype2-D array transducers for medical ultrasonic imaging, but the resultingproducts were acoustically unsuitable for modern medical ultrasoundimaging procedures.

Two dimensional arrays also have been confronted with the problem ofhigh electrical impedance in transducer elements. Two-dimensional arrayshave been developed in two geometries. A typical geometry for a 4×32array transducer is designed for focusing (but not steering) in theelevation direction and for correction of phase aberrations in twodimensions. Such transducers have been called 1.5-D arrays. For atransducer array of this design, each element typically exhibits compleximpedance, the magnitude of which is approximately 1000Ω at a resonanceof 3.5 MHz; this complex impedance causes an electrical impedancemismatch and the accompanying sensitivity decrease which are more severethan seen in linear arrays. Elements in full 2-D arrays which can steerthe ultrasound beam in azimuth as well as elevation may be smaller than0.2 mm×0.2 mm; these elements exhibit a complex electrical impedancehaving a magnitude of approximately 5000Ω or greater, so sensitivity isfurther reduced. Thus, for 1.5-D and 2-D arrays, the development ofsuitable piezoelectric materials are critical to improved sensitivity.

Unfortunately, the piezoelectric ceramics as described in the prior artare ill suited for such transducers. 1.5-D and 2-D arrays are commonlyfabricated by dicing a single piezoelectric chip in two directions witha kerf width as small as 0.01 mm.

In an attempt to address the problem of high electrical impedance inlinear arrays, U.S. Pat. No. 4,958,327 to Saitoh et al., teaches theconcept of a multi-layer ceramic piezoelectric material consisting of Klayers laminated in parallel electrically but in series acoustically.For K layers of uniform thickness, the capacitance of each element isincreased by K² ; this capacitive increase reduces the electricalimpedance of the element by K² significantly improving transmitefficiency and receive mode sensitivity. However, the teaching of Saitohis inapplicable to two dimensional arrays; as the electrode layers areshort circuited on a side surface of an element, the concept is limitedto elements with electrode layers having a surface on the periphery ofthe transducer, and cannot be used for the elements of the inner rows oftwo dimensional arrays.

Transducers may be developed using a piezoelectric substrate fabricatedfrom a composite of a piezoelectric ceramic phase such as PZT and aninert phase such as a polymer epoxy. However, piezoelectric compositesproduce a lower relative dielectric constant than PZT alone due to thepresence of the epoxy phase. The lower dielectric constant results in alower capacitance and thus a higher electrical impedance than PZT. Thehigher electrical impedance has limited the use of PZT/epoxy compositesin the small elements of steered linear phased arrays and twodimensional arrays. Moreover, the lower dielectric constant exacerbatesthe high impedance problem of two dimensional array elements describedabove.

In view of the foregoing, it is an object of the present invention toprovide a transducer array having decreased acoustic impedance,decreased electrical impedance and suppressed lateral mode.

An additional object of the present invention is to provide a method ofmaking a transducer chip having decreased acoustic impedance, decreasedelectrical impedance and suppressed lateral mode.

It is a further object of the invention to provide an ultrasoundtransducer array which contains such a transducer chip.

It is an additional object of the invention to provide ultrasounddiagnostic devices which utilize a transducer array as described.

SUMMARY OF THE INVENTION

These and other objects are satisfied by the present invention, whichincludes as a first aspect a piezoelectric transducer chip comprising aplurality of transducer elements arranged in a two-dimensional array. Atleast one of the transducer elements is a multi-layer element whichcomprises a plurality of piezoelectric layers, each of which isseparated from the adjacent piezoelectric layers by an electrode layerso that a plurality of capacitive elements are electrically connected inparallel. The piezoelectric layers may also be comprised of a pluralityof piezoelectric members wherein the members are separated from adjacentmembers to provide a gap between adjacent piezoelectric members andthereby form a piezoelectric composite layer. A first via connects afirst set of alternating electrode layers, and a second via connects asecond set of alternating electrode layers. The first via is insulatedfrom the second set of alternating electrode layers, and the second viais insulated from the first set of alternating electrode layers. Atleast one of the plurality of multi-layer elements has an internal edge.At least one of the vias of a multi-layer element is an internal via.

A second aspect of the present invention is an ultrasonic transducerarray utilizing a two-dimensional piezoelectric chip as described above.The transducer array comprises the 2-D chip, a connector having an arrayof connector pads for electrically connecting the connector to the chip,means for electrically connecting a first set of alternating electrodelayers of the chip to ground, and means for electrically connecting asecond set of alternating electrode layers of the chip to acorresponding one of the connector pads.

A third aspect of the invention is an ultrasonic scanner utilizing the2-D chip. The scanner comprises means for producing an ultrasonicsignal, an ultrasonic transducer array which includes the 2-D chip,means for amplifying a received ultrasonic signal, and means forprocessing and displaying the ultrasonic signal. In a preferredembodiment, the scanner is a medical diagnostic tool for ultrasonicscanning of tissue.

A fourth aspect of the present invention is a multi-layer ultrasonictransducer array made of a piezoelectric composite.

A fifth aspect of the present invention is a method of making amulti-layer ultrasonic transducer comprised of a piezoelectric compositematerial.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side cross-sectional view of a 3×3 two dimensionaltransducer array with five piezoelectric layers.

FIG. 2 is a top view of a 3×3 two dimensional transducer array.

FIG. 3A is a top section view of a two dimensional array having splitvias taken through an electrode layer of a first set of alternatingelectrode layers.

FIG. 3B is a top section view of a two dimensional array having splitvias taken through an electrode layer of a second set of alternatingelectrode layers interposed between the first set of electrode layers ofFIG. 3A.

FIG. 4 is a side cross-sectional view of a 3×3 two dimensionaltransducer array with two elements having five piezoelectric layers andone single layer element.

FIG. 5 is a cross-sectional view of a two-dimensional array including anupper stand-off for improved image quality obtained by direct contact ofthe transducer on a subject's skin.

FIG. 6 is a top view of the footprint of a transducer to be incorporatedinto a handle for easier use.

FIG. 7 is a bottom view of the transducer of FIG. 6 showing widerinterelement spacing.

FIG. 8 is a block diagram showing the electrical connections of anultrasonic scanner for medical diagnostic use.

FIG. 9 is a cross-sectional view of a transducer element according tothe present invention prior to heat curing the piezoelectric composite.

FIG. 10 is a cross-sectional view of a piezoelectric compositetransducer element according to the present invention.

FIG. 11 pictorially represents a piezoelectric ceramic layer at variousstages of fabrication.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a two-dimensional ultrasound transducerarray having improved sensitivity with small transducer elements. Itdoes so by providing a transducer chip with multiple piezoelectriclayers which can be fabricated using multi-layer ceramic (MLC)technology. Alternate piezoelectric layers are electrically connectedthrough the use of "vias."

An exemplary embodiment of a transducer chip of the present invention isillustrated in FIGS.1 and 2, which show schematically a transducer chip10 comprising a 3×3 array of transducer elements. Those skilled in thisart will recognize that the invention is not restricted to arrays ofthis size, but can include any two dimensional transducer array, whichas used herein refers to a transducer having a plurality of elementsarranged in a plurality of rows (N) and a plurality of columns (M) in arectangular N×M grid.

FIG. 1 shows three adjacent transducer elements 11, 12, and 14, each ofwhich is separated from the adjacent transducer element by a kerf 13,15. Each kerf 13, 15 can be filled with air or with some filler materialsuch as polymers, epoxies, glass balloons, plastic balloons, and thelike. Each transducer element 11, 12, 14 is electrically connected to acorresponding connector pad 16, 18, 20. Each of the transducer elements11, 12, and 14 comprises five piezoelectric layers (illustrated forelement 11 as layers 24, 26, 28, 30, 32) arranged in a vertical stack.Those skilled in this art will appreciate that although fivepiezoelectric layers are shown herein, any number of piezoelectriclayers can be included. Each of the piezoelectric layers 24, 26, 28, 30,32 is separated from its adjacent layers by an electrode layer(illustrated as layers 34, 36, 38, 40. An additional electrode layer 42contacts the lower surface 25 of the lowermost piezoelectric layer 24,and also contacts the connector pad 16. Likewise, an additionalelectrode layer 44 contacts the upper surface 33 of the uppermostpiezoelectric layer 32, and also contacts a ground plate 22 which coversall of the transducer elements.

Alternate electrode layers 42, 36, and 40 are electrically connected bya "via" 46, which as used herein is an electrical connection whichextends through an aperture in the layers of a multilayer substrate toelectrically connect certain of the layers of the element. In thepresent embodiment, the via 46 extends from just beneath electrode layer44 to electrode layer 42 and contacts electrode layers 36 and 40,thereby connecting these layers. The via 46 is insulated from connectionwith electrode layers 34, 38, and 44 by insulation gaps 48, 50 and 52.In the same manner, via 54 connects electrode layers 34, 38, and 44 byextending along an internal surface 61 of the transducer element 11 fromelectrode layer 44 to an insulation gap 56 which insulates the via 54from electrode layer 42. As used herein, an "internal surface" of atransducer element is a surface which adjacently faces another elementin another row or column across the kerf 13, as opposed to serving asthe peripheral surface of the array and thus being easily accessible forconnection. Insulation gaps 58 and 60 insulate the via 54 from theelectrode layers 36 and 40 respectively.

The two-dimensional nature of the transducer chip 10 can be best seen inFIG. 2, which shows the chip at the depth of electrode layer 44. Thevias 54 each extend to and thus electrically connect this electrodelayer. In contrast, the presence of the insulation gaps 52 prevents thevias 46 from electrically connecting with this electrode layer. The sameconfiguration would be seen at electrode layers 34 and 38. For electrodelayers 42, 36, and 40, the insulation gaps 56, 58, and 60 preventelectrical connection between the vias 54 and these electrode layer, butvias 46 do connect these electrode layers. Although they are shownherein to be located on the internal surfaces of the transducerelements, it is to be understood that the vias of the present inventioncan be prepositioned during fabrication so they can be located at anydesired point on each element, such as an internal surface or evenwithin the interior volume of the element. As used herein, a via whichis located on an internal surface or within the interior volume of anelement is referred to as an "internal via."

The interconnection of the first set of alternating electrode layers 42,36, 40 by a first via 46 and the interconnection by a second via 54 of asecond set of alternating electrode layers 34, 38, 44 interposed betweenthe first set of alternating electrode layers provides an element whichcomprises five capacitive elements connected in parallel. As a result,the capacitance of the total transducer element 11 is increased over asingle piezoelectric layer of the same thickness as the total stack bythe square of the number of layers; i.e., in this instance by 5² =25.Accordingly, the impedance of this element is reduced by that samefactor, which improves the impedance match of these elements toelectrical sources to which they are typically attached.

As those skilled in this art will appreciate, the number and thicknessof piezoelectric layers in an element can vary depending on thecharacter of the connecting device. In a preferred embodiment, thepiezoelectric layers of a transducer chip 10 can be between about 0.01and 0.15 mm in thickness, and more preferably can be between about 0.02and 0.06 mm in thickness. As an example, to achieve a resonant frequencyof 2.5 MHz in a conventional 2-D array transducer, a PZT chip ofthickness of about 0.6 mm is required. A typical MLC-producedpiezoelectric layer thickness is 0.04 mm after sintering, so K=11 layerscan be easily included in a 2.5 MHz chip. Thus, the capacitance of eachlayer of the MLC element would be increased by a factor of 11 and thecapacitance of the complete stack of 11 capacitors in parallel for asingle element will be increased by K² =11² =121. The element impedanceis then reduced by a factor of 121 from 5KΩ to 41Ω, an excellent matchto a 50Ω electrical source.

The present invention may be used over a wide range of operatingfrequencies of from about 1 MHz to about 10 MHz and above. The physicaldimensions and number of elements in the two-dimensional array willdepend upon the application of the transducer array. For example, asquare array of square transducer elements can be utilized for threedimensional imaging systems. Square transducer elements of from about0.05 mm to about 1 mm are suitable for three dimensional imaging usingfrequencies of from about 10 MHz to about 1 MHz. However as smallerdimensions are utilized, operating frequencies of greater than 10 MHzmay be achieved. The desired frequency of ultrasound determines theheight of the chip; for example, for a 20 MHz signal, the chip can be≈0.05 mm, and for a 1 MHz signal, the chip can be ≈1 mm. The thicknessof the chip then determines the depth of the kerf.

In an alternative embodiment, each circular via in FIG. 2 can be split,as shown in FIGS. 3A and 3B, which illustrate electrode layers 34, 38,44 (FIG. 3A) and electrode layers 42, 36, 40 (FIG. 3B). The result is atransducer chip wherein a single circular via 54 can independentlyconnect electrode layers 34, 38, 44 on the left edge of element 11 (FIG.3A) as well as independently connecting layers 42, 36, 40 on the rightedge of element 12 (3B). In this configuration, a single via is able toserve two independent elements. This design feature of splitting eachvia offers the additional advantage that one side of each split via isgrounded so that two signal vias are not immediately adjacent separatedonly by the saw kerf. This design will reduce electrical cross talk inthe piezoelectric MLC.

In an alternative embodiment to the foregoing, for fabrication reasonsit may be advantageous for a split via, rather than having one half ofthe split via in contact with the electrode layer of one element and theother half of the split via insulated from the same electrode layer onthe adjacent element, instead to comprise halves which are mirror imagesof one another about a plane defined by the center of the kerfseparating the adjacent elements. Thus in FIG. 3A, via 54 would be incontact with electrode layer 34 on both element 11 and element 12. Splitvia 55, located on the other (leftmost in FIG. 3A) surface of element12, would then be insulated from electrode layer 34 by an insulationgap. This pattern would continue for the other electrode layers 38, 44in contact with electrode layer 34 and for other elements of the array.In a similar fashion, for electrode layer 36 shown in FIG. 3B, the halfof split via 54 associated with element 12 (the left half in FIG. 3B)would be insulated from electrode layer 36 by an insulation gap whichmirrors that shown for element 11. On the leftmost edge of element 12,the split via would be in contact with the electrode layer. The samepattern would be followed for electrode layers 42, 40.

An additional alternative embodiment is schematically illustrated inFIG. 4 as a three-element two-dimensional array. A transducer chip 100comprises two multilayer elements 110, 120 similar in configuration tothose described above, and an element 130 comprising a singlepiezoelectric layer 130. By mixing a plurality of single layer elementsand plurality of multilayer elements within the same two-dimensionalarray, the pulse-echo sensitivity of the transducer can be improvedfurther.

Fabrication of a MLC piezoelectric chip, which is based on computeraided design, proceeds as follows. PZT powder is mixed with organicbinders, plasticizers, and solvents to form a slurry. The slurry isspread to form a thin layer and heated to form a so-called "green tape."Slurry thickness is controlled using a doctor blade technique; exemplaryis a green tape thickness of between 0.05 and 0.15 mm. Multiple holesare punched (mechanically or by laser), drilled, or etched into the tapeto form the vias on each layer. The via holes are filled with a metalpaste (e.g. silver or platinum) and the surface electrodes (silver orplatinum paste) for each layer are laid down by screen printingexcluding the insulation gaps. Multiple layers of green tape are thensuperimposed to align the vias, the multi-layer sandwich is laminatedand then finally sintered to form a single package. Metallization isthen plated or vacuum deposited on the input pads.

This transducer chip 10 then can be attached to a substrate containingelectrical contact pads 16, 18 and 20 using any number of methods ofbonding techniques. One such bonding technique uses conductive epoxy fora resistive contact. Another bonding technique uses a thin filmapproximately 1 micron thick of nonconductive epoxy for a capacitivecontact. The electrical contact pads are connected to wires or vias in amulti-layer ceramic connector, traces on a circuit board, or flexiblepolymer circuit material. Optional conductive films can be depositedonto the piezoelectric chip 10 to produce a plurality of λ/4 matchinglayers to tissue.

This structure can then be divided into a plurality of transducerelements by any procedure which creates separate piezoelectric elements,such as dicing with a dicing saw. Dicing may be carried out using K & SDiamond Wheel Dicing Saw which produces kerf widths about 25 microns.The size and shape of the transducer elements is determined by thedicing pattern and is typically a square or checkerboard pattern.However other patterns such as parallelograms, circles and rhombuses maybe used depending upon the specific application of the transducer array.The actual configuration of the transducer array, however, may beselected by selectively establishing electrical connections to specifictransducer elements in the checkerboard, by selective placement ofconnector pads or vias or by other electrical means. Active transducersmay be configured by virtue of said selective connections in any numberof predetermined patterns such as a cross, a filled or unfilledrectangle or a filled or unfilled circle. Note that through selection ofactive transducer elements, the patterns for the send transducers may bethe same or different from the pattern for the receive transducers. As afinal step, the ground plate 22, usually a conductive foil, is thenbonded to the piezoelectric chip 10 with a bonding agent.

An exemplary chip of the present invention formed by this method is 16layers of PZT-5A, each layer being 0.08 mm thick, to yield a stackthickness of 1.3 mm (assuming 20 percent shrinkage during sintering).This stack has a resonant frequency of about 1.0 MHz.

FIG. 5 shows an alternate embodiment of the present invention whichincludes a stand-off 100 to allow improved use of the present inventionfor medical imaging applications by allowing improved contact with theskin surface of a patient for small acoustic windows on the body such asthe inter-costal space between the ribs for cardiac ultrasounddiagnosis. This stand-off may be fabricated using conventionaltechnology.

Optionally, the two-dimensional array ultrasonic transducer of thepresent invention may have means for redistributing the electricalconnections of the connection pads 16, 18, 20 so as to increase thedistance between electrical connections to a greater distance than thatbetween individual connector pads 16, 18, 20. This increase in spacingbetween electrical connections allows for simpler connection to externalelectronics such as voltage sources and input amplifiers. The increasedspacing allows for the use of coaxial connections between the transducerarray and the external electronics which results in reduced noise in theelectrical output from the transducer and thereby increases the usablesensitivity of the transducer array. The increased spacing isaccomplished through conventional bond wiring, circuit boards, flexiblepolymer circuits, or the use of MLC technology which is illustrated anddescribed in co-pending U.S. patent application Ser. No. 07/883,006, theentirety of which is incorporated herein by reference.

The two dimensional array ultrasonic transducer of the present inventionmay also be incorporated into a handle for easier use in medical andother applications. An example of the top view of the transducer isshown in FIG. 6, in which the interelement transducer spacing is 0.2 mmso that the total footprint on the skin surface is only a 5 mm×5 mmsquare. FIG. 7 shows the bottom view of the transducer of FIG. 6 andshows a flange containing a pad grid array for connection to an optionaltransducer handle. The inter-element spacing of the pads is 0.635 mm sothat a redistribution, or fan-out, occurs in the connector, therebyenabling easier electrical connection to the cables of the transducerhandle.

Uses for the present invention include three dimensional ultrasoundimaging or volumetric measurements and thin slice ultrasound imaging. Inuse, the transducer elements of the two-dimensional array ultrasonictransducer are excited by a voltage source in electrical connection withthe transducer elements through the connector. The electrical voltagesource places an electrical voltage across the element to produce anultrasonic output from the element. These voltages typically range fromabout 50 volts to about 300 volts. The voltage excites the transducerelement to produce an ultrasonic signal which is transmitted from thetransducer array into a test region. When receiving ultrasonic signals,the ultrasonic signal excites a transducer element to produce anelectrical voltage across the transducer element. This electricalvoltage is then amplified by an amplifier in electrical connection withthe transducer element through the connector. A further advantage of thepresent invention is the ability to use what is known in the art as"cavity down" positioning of an integrated circuit with the connector toprovide amplifiers for receive and transmit mode use of the transducersin a single integrated package. Using the "cavity down" method, anintegrated circuit is mounted directly onto the connection side of themultilayer ceramic connector thereby incorporating the integratedcircuit as part of the transducer array assembly and allowing for theintegration of the circuitry into the handle of the transducer array toprovide a more compact unit.

FIG. 8 shows a block diagram of a phased array medical ultrasonicscanner 140. The scanner includes transmitter circuitry 150, atransducer array 160 of the present invention, receiver amplifiercircuitry 170, signal processing circuitry 180, such as envelopedetection and filtering, a scan converter 190, and a television monitor200.

Other aspects of the present invention will now be described withreference to the two-dimensional multi-layer transducer array describedabove. However, as will be appreciated by one of skill in this art, theadvantages and benefits of the following embodiments are also applicableto linear and 1.5D arrays. Thus, linear arrays with reduced acousticimpedance and suppressed lateral mode may be made through the use ofmultilayer elements of piezoelectric composite materials. Thus, as willbe appreciated by those of skill in this art, greatly simplifiedmanufacturing techniques and transducer array layouts may be used toproduce, for example, linear transducer arrays of multi-layerpiezoelectric composite materials while still obtaining the benefits andadvantages of the present invention.

In one embodiment of the present invention, a two dimensional ultrasonictransducer array is comprised of a piezoelectric composite material. Inthe fabrication of two dimensional ultrasonic transducer arrays, thecritical descriptor of piezoelectric composites is the so calledconnectivity which describes the self connection of each phase in 1, 2,or 3 dimensions. Previously, the best performance of a PZT/epoxycomposite for medical ultrasound transducers uses a 1-3 connectivity,i.e., a regular array of PZT members aligned in the thickness dimensionof the transducer (connectivity=1) surrounded by a matrix of epoxy(connectivity=3).

Currently, the limiting factor in fabrication is the minimum PZT membersize which presently results in an array of members 0.025 mm in diameterso that even a small array element 0.2 mm×0.2 mm would allow as many as5×5=25 members per element. For a piezoelectric ceramic component ofPZT-5H with a relative dielectric constant of ε^(s) ₃₃ =1430, onearrives at a dielectric constant of approximately 800 for a composite of50% volume fraction of PZT combined with 50% epoxy. This low dielectricconstant exacerbates the high electrical impedance problem of twodimensional array elements. Thus the number of layers required toachieve a low electrical impedance is increased for this examplecomposite material by the ratio of 1430/800, relative to that of a PZTMLC. PZT/air composite may also be used. In this case, for PZT-5H, therelative dielectric constant of a 50% PZT/50% air composite is 715, andthe number of layers required in an MLC 2-D array is increased by afactor of 2. For example, we have recently fabricated a 1.5D arraytransducer from MLC PZT-5H material including electrical vias asdescribed above. The array included 3×43=129 elements and consisted of 3layers each 0.22 mm for a total stack thickness 0.66 mm. If the arraywere fabricated from a 1-3 connectivity composite (50% volume fractionPZT/50% air), a 6 layer stack would be required with each layerthickness=0.11 mm. For 1.5D arrays, typical layer thicknesses are fromabout 0.01 mm to about 0.3 mm.

There are several alternate implementations to fabricate a multi-layertwo dimensional array of the 1-3 connectivity composite material.However, it should be noted that in the firing and sintering processesfor the MLC materials, temperatures approaching 2000° C. are often usedto achieve a rigid structure. Thus all organic components are burned offleaving essentially a PZT/air composite.

FIG. 9 illustrates a cross-section of a single element in a twodimensional array composed of a 3 layer 1-3 connectivity piezocompositeof 50% PZT/50% air. The simplest method of producing such an MLCmaterial, as shown in FIG. 9, is to make each layer an independent 1-3connectivity composite by simply using a PZT particle 300 which isapproximately the size of the green tape layer thickness whilemaintaining a 50% volume fraction of PZT. As seen in FIG. 9, the PZTparticles 300 are placed between electrode layers 330, 340 with an airgap or organic filler 350 separating adjacent particles. Alternatingelectrodes are interconnected by a ground via 310 and a signal via 320to produce the multilayer element as described above. While FIG. 9illustrates the placement of the PZT particles 300 as a cross-section ofthe multiple layers, the PZT particles 300 may actually be distributedin a two-dimensional pattern for a given layer. After drilling the vias,electroding, laminating, firing and sintering, and poling, usingconventional MLC fabrication techniques, each layer in the MLC stackexhibits the properties of the 1-3 connectivity PZT/air composite sinceeach layer is independently electroded.

In a more labor intensive fabrication method, one can use a conventionaldicing and filling technique to produce the PZT members in each greentape layer. Laser milling or jet machining may be used to form themembers in the ceramic green tape before backfilling with epoxy.However, it should be noted that in both these methods, there is noalignment or connectivity of PZT particles or members between layers.

FIG. 10 illustrates the cross-section of a completed composite elementof a two-dimensional transducer array. As seen in FIG. 10, thepiezoelectric layer is comprised of a plurality of piezoelectric members400 separated by a gap 350 which may be filled with air or an organicfiller. Suitable fillers for the gap 350 include epoxies, poly methylmethacrylate in cellulose acetate, poly vinyl alcohol or thecommercially available organic binder Cladan produced by Tam Ceramics,Inc. of Niagra Falls, N.Y. The piezoelectric member 400 may be apiezoelectric rod and may be rectangular, circular, elliptical or othersuitable cross-sectional shape. As FIG. 10 further illustrates, thepiezoelectric members 400, 410, 420 are aligned across the layers of themultilayer element. Furthermore, as described above, these piezoelectricmembers 400, 410, 420, may be arranged in a two-dimensional pattern.However, to allow for alignment of the piezoelectric members, thetwo-dimensional pattern of the members should be the same from layer tolayer or misalignment may occur. The layers of piezoelectric members areseparated by the signal electrodes 340 and the ground electrodes 330 asdescribed above to form the multilayer transducer element.

Alignment or registration of the members of a 1-3 connectivity PZTcomposite between layers of the MLC can be achieved by using the diceand fill fabrication method on each individual layer of PZT ceramic.This process may be used on individual PZT plates or on PZT green tapeswhich have undergone a preliminary firing process to make the greentapes more rigid. Before lamination each PZT layer is diced by a diamondwheel, dicing saw, jet machining, or laser scribed and filled with epoxyor an organic material using the conventional techniques to obtain thepost patterns shown in FIG. 10. In one technique, two sets of deepgrooves are cut in a block of piezoceramic at right angles to eachother. A polymer is then cast into these grooves and the solid ceramicbase is ground off. After polishing the plate to final thickness,electrodes are applied to the faces and the ceramic is poled by applyinga strong electric field, usually at slightly elevated temperatures.FIGS. 11(a) through 11(h) illustrate an alternative method of formingthe piezoelectric members. As seen in FIG. 11, to create thepiezoelectric posts for a particular layer, a ceramic plate of thedesired thickness (FIG. 11(a)) may be diced with two sets of groovesperpendicular to each other (FIGS. 11(b) and 11(c)). These grooves onlyextend partially through the ceramic plate. A polymer is then vacuumcast into the grooves (FIG. 11(d)) with a lid (not shown) waxed onto thetop of the ceramic to prevent any polymer from coating the tops of thepillars. The ceramic plate is then turned over (FIG. 11(e)) and theprocess repeated with the grooves on the opposite side of the ceramicplate being aligned with the first set of grooves (FIGS. 11 (f) and11(g)). The second set of grooves are then vacuum cast with a polymer(FIG. 11(h)) and the capping plates (not shown) are removed.

After fabrication of the piezoelectric members for a given layer,electrodes are screen printed and fired and vias are drilled for eachlayer. The PZT posts of one layer are then aligned with those of thenext layer using the same alignment techniques and registration holesnormally used to align the vias of conventional multilayer ceramics.Finally, the stack is laminated to obtain the 1-3 connectivity PZT/aircomposite MLC stack with PZT posts aligned between layers.

It should be noted that there are also lower temperature processes tofabricate multi-layer ceramic connectors in the electronic industryusing firing temperatures below 1000° C. Applying these processes, itwill be possible to use an inert material including a perfluoroalkylpolymer such as Teflon® or Zonal sold by E. I. Dupont between the PZTposts which will not burn off during the firing, laminating, sinteringsteps. Organic binders may also be deposited in the gap by vacuumimpregnation after heating These composites will exhibit higherdielectric constant than a PZT/air composite. In an alternateimplementation, a sheet of polyimide substrate commonly used inmicroelectronic connectors is etched by conventional photolithographicmethods or by conventional reactive ion etching in the desired patternleaving a cavity in the polyimide for each PZT post. A fine grain slurryof PZT is used to fill the cavities with the polyimide. Vias andelectrodes for a two dimensional array transducer are implemented as inconventional thin film technology for polyimide substrates, i.e.,sputtering or vacuum deposition. The vias and PZT are aligned betweenmultiple identical layers, the layers are laminated and cured leaving a1-3 connectivity PZT/polyimide composite MLC.

In the above description, certain aspects of the present invention weredescribed with respect to the use of PZT as a piezoelectric material.However, the present invention is not limited to the use of PZT but mayalso utilize other piezoelectric materials or ferroelectric materials.Certain aspects of the present invention have also been described withrespect to 1-3 connectivity material. However, the present invention mayalso utilize or produce other connectivity materials, such as 2-2 or 0-3connectivity materials.

The foregoing is illustrative of the present invention and is not to beconstrued as limiting thereof. For example, other methods of fabricationof the present invention may be utilized while still benefitting fromthe teachings of the present invention. Those skilled in this art willalso appreciate that other methods of increasing the distance betweenelectrical connections to the transducer elements of the presentinvention may be utilized. The invention is accordingly defined by thefollowing claims, with equivalents of the claims to be included thereof.

That which is claimed:
 1. A piezoelectric transducer chip comprising:aplurality of transducer elements, at least one of said transducerelements being a multilayer element comprising a plurality ofpiezoelectric layers and a plurality of electrode layers forming aplurality of capacitative elements electrically connected in parallel,each of said plurality of piezoelectric layers being separated from theadjacent piezoelectric layers by one of said plurality of electrodelayers, and said piezoelectric layers consisting of a first set ofalternating electrode layers and a second set of alternating electrodelayers interposed between said first set of alternating electrodelayers; and wherein each of said piezoelectric layers of said multilayerelement is a piezoelectric composite material comprising a plurality ofpiezoelectric members formed between adjacent electrodes of saidplurality of electrode layers each of said members being spaced apartfrom adjacent members to thereby form a gap between each of saidplurality of piezoelectric members.
 2. A piezoelectric transducer chipaccording to claim 1, wherein said plurality of transducer elementscomprises a plurality of multilayer elements.
 3. A piezoelectrictransducer chip according to claim 1, wherein said piezoelectric membersare PZT.
 4. A piezoelectric transducer chip according to claim 1,wherein the gap between said piezoelectric members is filled with amaterial selected the group consisting of air and organic binders.
 5. Apiezoelectric transducer chip according to claim 1, wherein at least oneof said transducer elements has a single piezoelectric layer.
 6. Apiezoelectric transducer chip comprising:a plurality of transducerelements arranged in a two-dimensional array, at least one of saidtransducer elements being a multilayer element having an internal edgeportion and comprising a plurality of piezoelectric layers and aplurality of electrode layers forming a plurality of capacitive elementselectrically connected in parallel, each of said plurality ofpiezoelectric layers being separated from the adjacent piezoelectriclayers by one of said plurality of electrode layers, and saidpiezoelectric layers consisting of a first set of alternating electrodelayers and a second set of alternating electrode layers interposedbetween said first set of alternating electrode layers; wherein each ofsaid piezoelectric layers of said multilayer element is a piezoelectriccomposite comprising a plurality of piezoelectric members formed betweenadjacent electrodes of said plurality of electrode layers each of saidmembers being spaced apart from adjacent members to thereby form a gapbetween each of said plurality of piezoelectric members; a first viaelectrically connecting said first set of alternating electrode layers,said first via being electrically insulated from said second set ofalternating electrode layers; and a second via electrically connectingsaid second set of alternating electrode layers, said second via beingelectrically insulated from said first set of alternating electrodelayers; wherein at least one of said vias is an internal via.
 7. Apiezoelectric transducer chip according to claim 6, wherein saidplurality of transducer elements comprises a plurality of multilayerelements.
 8. A piezoelectric transducer chip according to claim 6,wherein said piezoelectric members are PZT.
 9. A piezoelectrictransducer chip according to claim 6, wherein the gap between saidpiezoelectric members is filled with a material selected from the groupconsisting of air and organic binders.
 10. A piezoelectric transducerchip according to claim 6, wherein said first via is insulated from saidsecond set of alternating electrodes by a plurality of insulating gaps.11. A piezoelectric transducer chip according to claim 6, wherein saidsecond via is insulated from said first set of alternating electrodes bya plurality of insulating gaps.
 12. A piezoelectric transducer chipaccording to claim 6, wherein at least one of said transducer elementshas a single piezoelectric layer.
 13. An ultrasonic transducer arraycomprising:(a) a connector having an upper surface, a lower surface, andan array of connector pads formed in said connector for electricallyconnecting said upper surface to said lower surface; (b) a piezoelectrictransducer chip having a plurality of transducer elements arranged in atwo-dimensional array, at least one of said transducer elements being amultilayer element having an internal edge portion and comprising aplurality of piezoelectric layers and a plurality of electrode layersforming a plurality of capacitive elements electrically connected inparallel, each of said plurality of piezoelectric layers being separatedfrom the adjacent piezoelectric layers by one of said plurality ofelectrode layers, and said piezoelectric layers consisting of a firstset of alternating electrode layers and a second set of alternatingelectrode layers interposed between said first set of alternatingelectrode layers and wherein each of said piezoelectric layers of saidmultilayer element is a piezoelectric composite material comprising aplurality of piezoelectric members formed between adjacent electrodes ofsaid plurality of electrode layers each of said members being spacedapart from adjacent members to thereby form a gap between each of saidplurality of piezoelectric members; (c) a first via connecting saidfirst set of alternating electrode layers, said first via beingelectrically insulated from said second set of alternating electrodelayers; and (d) a second via connecting in parallel said second set ofalternating electrode layers, said second via being electricallyinsulated from said first set of alternating electrode layers; whereinat least one of said vias is an internal via; (e) means for electricallyconnecting said first set of alternating electrodes to ground; and (f)means for electrically connecting said second set of alternatingelectrodes to a corresponding one of said connector pads.
 14. Anultrasonic transducer array according to claim 13, which furthercomprises a metallic ground sheet which overlies said piezoelectrictransducer chip and is electrically connected to said means forelectrically connecting said first set of alternating electrodes.
 15. Anultrasonic transducer array according to claim 13, wherein said meansfor electrically connecting said second set of alternating electrodescomprises a bonding layer to connect to said upper surface of saidconnector.
 16. An ultrasonic transducer array according to claim 13,wherein said piezoelectric chip comprises a plurality of multilayerelements.
 17. An ultrasonic transducer array according to claim 13,wherein said piezoelectric chip comprises at least one multilayerelement which includes a first internal edge and a second internal edge,and wherein said first via is an internal via, and wherein said secondvia is an internal via.
 18. A piezoelectric transducer chip according toclaim 13, wherein said piezoelectric members are PZT.
 19. Apiezoelectric transducer chip according to claim 13, wherein the gapbetween said piezoelectric members is filled with an organic binder. 20.An ultrasonic scanner comprising:(a) means for producing an ultrasonicsignal; (b) an ultrasonic transducer array of claim 1 operativelyconnected to said means for producing an ultrasonic signal fortransmitting said ultrasonic signal to a target; (c) means foramplifying a received ultrasonic signal from a target operativelyconnected to said ultrasonic transducer array; and (d) means forprocessing said ultrasonic signal operatively connected to saidamplifying means.
 21. A method of making a two-dimensional transducerchip having at least one multilayer transducer element of apiezoelectric composite material, said method comprising:forming a firstpiezoelectric layer comprising a plurality of members of piezoelectricmaterial wherein each member of said plurality of members is spacedapart from adjacent members to thereby form a gap between saidpiezoelectric members; forming a second piezoelectric layer comprising aplurality of members of piezoelectric material wherein each member ofsaid plurality of members is spaced apart from adjacent members tothereby form a gap between said piezoelectric members and wherein saidpiezoelectric members are positioned to form said pattern; forming afirst electrode layer on said first piezoelectric layer to electricallyconnect each of said piezoelectric members to the remainder of saidpiezoelectric members of said first piezoelectric layer; and attachingsaid second piezoelectric layer to said first electrode layer such thatthe piezoelectric members of said first piezoelectric layer are alignedwith the piezoelectric members of said second piezoelectric layer and sothat each of said piezoelectric members of said second piezoelectriclayer are electrically connected to the remainder of said piezoelectricmembers of said second piezoelectric layer and so that said firstpiezoelectric layer is electrically connected to said secondpiezoelectric layer.
 22. The method of claim 21, further comprising thesteps of:positioning said piezoelectric members of said firstpiezoelectric layer to form a pattern; and positioning saidpiezoelectric members of said second piezoelectric layer to form saidpattern.
 23. The method of claim 21, further comprising the step offilling the gap between said piezoelectric members with an organicbinder.
 24. The method of claim 21, further comprising the stepsof:forming an electrode layer on the free end of said firstpiezoelectric layer to electrically connect each of said piezoelectricmembers to the remainder of said piezoelectric members of said firstpiezoelectric layer; and forming an electrode layer on the free end ofsaid second piezoelectric layer to electrically connect each of saidpiezoelectric members to the remainder of said piezoelectric members ofsaid first piezoelectric layer.
 25. The method of claim 24 furthercomprising the step of selectively connecting said electrode layers soas to form a plurality of capacitive elements electrically connected inparallel.